Solid waste management represents one of the important areas of international requirements for health, environmental pollution control and economic development. Various waste products or materials are generated by various industries. For example, during Portland cement production, cement kiln dust (CKD) is removed from the stream of kiln gases as they pass through the kiln's dust collection system during clinker production. CKD poses a health hazard, storage problem, and is a potential source of pollution. Thus, in addition to being a waste of potentially valuable material, it also presents serious air stream pollution and dumping problems.
On the other hand, another waste by-product, which is produced in large amounts, is sulfur. Thus, large quantities of by-product sulfur are currently generated by the cleanup of hydrogen sulfide in the up-stream production of petroleum and natural gas and down-stream refinery operations. Such sulfur does have some uses, such as in the manufacture of fertilizers and certain chemicals. However, there has been a noticeable tendency towards an oversupply of sulfur resulting from the ever increasing desulfurization of primary products.
In the following, brief description of the waste by products is discussed.
CKD
CKD is a powder composed principally of micron-sized particles collected from electrostatic precipitators during the high temperature production of cement clinker. The chemical composition of CKD depends both on the raw materials used to produce the clinker, and on the type and source of carbon-based fuel used to heat the clinker in the rotary kiln. The raw material is a combination of calcareous rock or sediment, such as limestone or chalk; and an alumino-silicate material, such as clay or shale.
CKD can vary in composition from virtually unaltered kiln feed to over 90% alkali sulfates and chlorides depending on process type, kiln configuration, raw materials, fuels, process characteristics, and points of collection. It can vary in particle size from that of fine sand or silt to that of clay, with particle size distribution ranging from very broad to very narrow depending on material and process parameters. The quantities of dust generated from a particular kiln depends on the factors that control CKD composition as well as the internal configuration of the kiln, the quantities of gases passed through the kiln, and other operating conditions.
CKD is a major problem at many cement manufacturing plants. Dust is generated in large quantities and is often not suitable for direct return to the cement-producing process as a feed because of high concentrations of alkali metals and sulfates, and incompatibility of the dust with the process. Since large quantities of dust cannot be returned directly to the kiln, it must be disposed of in a safe manner. General disposal practices are placement of dust in waste piles or in land- or quarry fills. Such disposal methods are inherently unsatisfactory because they involve wasting a material for which significant processing and handling costs and efforts have been incurred. Since environmental regulations have matured, the costs and problems of disposal have become more onerous and continued disposal of kiln dust has become more expensive.
When CKD is brought into contact with water, high concentrations of anhydrous phases, which include oxides, sulfates and chlorides, are soluble and leached. Since the prime source of CKD instability is the high contribution of alkali metals oxides, and sulfates, which have high affinity towards water, the question is how to convert the undesired oxides into stable materials like carbonates or bicarbonates, to decrease the solubility, and consequently increase the durability of the application, through effective and inexpensive processes.
Treated CKD has the potential for use in engineering projects such as soil stabilization, waste stabilization/solidification, Portland cement replacement, asphalt pavement, controlled low strength material (flowable fill), Pozzolanic activator, lightweight aggregate, and construction fill, but this isn't always possible
Problems relating to CKD have long been recognized, and various methods have been proposed for their solution. The following methods have been suggested for treating CKD. The methods include leaching the dust with water to remove alkalis. The hazard potential of CKD can also be reduced by converting the chemical constituents into an insoluble and immobile form, i.e., stabilization. This involves chemical changes to the stable constituents in the treated substance to produce insoluble, immobile and less toxic compounds.
Nestell, in a U.S. Pat. No. 1,307,920 mixed kiln dust with water and passed carbon dioxide into the resulting mixture to substantially neutralize the slurry. However the product could not be recycled back into the cement kiln for its use as a kiln feed material unless the alkali levels of the original dust were very low.
Palonen et al., in U.S. Pat. No. 2,871,133 agglomerated CKD at high pressure and temperature, to render the alkalis more soluble. The resulting heat-treated agglomerate was then leached with water to remove the soluble alkalis. The residual solids are further treated to adjust moisture for return to a cement kiln. This process suffers because it is very complicated.
Patzias, in U.S. Pat. No. 2,991,154 mixed kiln dust with water and then heated at a known pressure. The slurry was filtered to separate the solution containing the alkalis from the residual solids. Then the separated solution was treated by neutralization with sulfuric acid, evaporation, centrifugation, or a combination thereof, to recover alkali sulfates, for recycling to the cement-making process. This process is not practical because of high water to dust ratio, high temperature, and high pressure to affect the dissolution of alkalis. Kiln dust solids would differ significantly in composition from normal kiln feed requiring kiln feed correction.
McCord, in U.S. Pat. No. 4,031,184 leached CKD at high temperature (but not at high pressure) using potassium chloride to enhance solubility. Then, the CKD solids are flocculated using oil and a fatty acid and the precipitates are palletized. Since the solubility of potassium chloride is higher than that of potassium sulfate by more than a factor of two in both hot and cold water, it is much more likely that any precipitate will be potassium sulfate rather than potassium chloride.
Helser, et al, in U.S. Pat. No. 4,219,515 added carbon dioxide to wastewater from the production of hydrous calcium silicates from lime and silica in order to remove calcium from the water so that it can be recycled to the production process. The resulting calcium carbonate precipitate presumably can be re-producing lime.
Kachinski, in U.S. Pat. No. 4,402,891 added water to CKD in a carbon dioxide atmosphere. Alkalies are not completely removed, and the material is not suitable for return to a cement-making process.
Neilsen, in U.S. Pat. No. 5,173,044 used wet-process slurry to scrub sulfur from kiln gases and retain them in the kiln. This process is of limited applicability because it retains all of the alkalies in the kiln so that, in the majority of cases, only limited amounts of CKD can be used.
Brentrup, in U.S. Pat. No. 5,264,013 collected CKD in a conventional dust collector, which was later progressively heat-treated to volatilize low-boiling pollutants and collect them with a carbonaceous filter medium. The ability to return CKD to the cement-making process was not enhanced.
Huege, in U.S. Pat. No. 5,792,440 used carbon dioxide to treat a supernatant liquid after leaching and separation of the solids from lime kiln dust in order to produce high purity precipitated calcium carbonate as a separate product, for treating flue gases exhausted from a lime kiln. This method is only useful as an effluent control.
Gebhardt, in U.S. Pat. No. 6,331,207 moistened the supply of CKD with carbon dioxide to convert the materials to carbonates. During the carbonation cycle, the water in the hydroxides is released to formulate slurry. The soluble alkalis and sulfate are released in the liquid phase with the solids being separated from the liquid. Then, the solids were washed to provide a useful feed to the kiln while, the liquid contains alkali salts.
Prior methods that have been used in the past frequently suffer from the following problems:                1. Only part of the alkalis are readily soluble, often half or less.        2. Typical ratios of water to dust are 10:1 to 20:1, or higher.        3. An effluent, high in pH (>10) and dissolved solids, is discharged.        4. Dissolved solids tend to precipitate in the receiving waters.        5. The high pH effluent is detrimental to the biosphere.        6. The recovered solids are high in water content, often over 70%.        7. Adjustments to kiln feed chemistry may be required when treated dust is returned to the kiln.        
These problems are so severe that the leaching methods of the past have been largely banned by Environmental Protection bodies.
Sulfur and SPC
On a different note, another waste by-product which is produced in large amounts is sulfur. Thus, large quantities of by-product sulfur are currently generated by the cleanup of hydrogen sulfide in the up-stream production of petroleum and natural gas and down-stream refinery operations. Such sulfur does have some uses, such as in the manufacture of fertilizers and certain chemicals. However, there has been a noticeable tendency towards an oversupply of sulfur resulting from the ever increasing desulfurization of primary products.
The U.S. Bureau of Mines developed techniques for using by-product sulfur to stabilize toxic and hazardous wastes (Sullivan, T. A. and Mc Bee, W. C., 1976, Development and testing of superior sulfur concretes, BuMines Report No. RI 8160, U.S. Bureau of Mines, Washington, D.C., 30; and Mc Bee, W. C., Sullivan, T. A. and Jong, B. W., 1981, Modified sulfur concrete technology, Proceedings, SULFUR-81 International Conference on Sulfur, Calgary, 367-388). Developments in sulfur polymer concrete (SPC) are in progress to find alternative markets for the excess in elemental sulfur. SPC typically consists of elemental sulfur, sulfur polymer stabilizer, fine filler material, and aggregates which can include waste materials such as sand, blast furnace slag, and fly ash (Kalb P. D., Heiser J. H., Colombo P., 1991, Modified sulfur cement encapsulation of mixed waste contaminated incinerator fly ash, Waste Management, pages 11:147; ACI Committee 548, 1993, Guide for mixing and placing sulfur concrete in construction [ACI 548.2R-93], American Concrete Institute, Farmington Hills, Mich., USA; Mohamed, A. M. O. and El Gamal, M. M., 2006, Compositional control on sulfur polymer concrete production for public works, in: “Sustainable Practice of Environmental Scientists and Engineers in Arid Lands, A. M. O. Mohamed [ed.], A. A. Balkema Publishers, 556 pages; Mohamed, A. M. O. and El Gamal, M., 2007a, “Sulfur based hazardous waste solidification”, Environmental Geology, Volume 53, Number 1, pages 159-175; and Mohamed, A. M. O. and El Gamal, M, 2007b, “Development of modified sulfur cement and concrete barriers for containment of hazardous waste in arid lands”, Sustainable Development and Climate Change”, Feb. 5-7, 2007, Doha, Qatar).
SPC can be produced by a hot mix procedure similar in some respects to that of asphalt. SPC is a construction material with unique properties and characteristics. It can perform well in some aggressive environments and can offer benefits as an alternative construction material, particularly in situations that require a fast setting time, placement in excessive cold or hot climates, corrosion resistance and impermeability.
CKD-Based SPC
SPC with excellent strength properties may be prepared from sulfur and CKD, however, material durability is a problem especially when the SPC is exposed to humid conditions, and failure is immanent. CKD is composed of an assemblage of oxidized and anhydrous phases, which are unstable or highly soluble at earth surface conditions. Undesirable components of CKD can escape from CKD-containing SPC into the surrounding environment and this can cause problems.